1. Field of the Invention
Aspects of the present invention relate to a cathode catalyst for a fuel cell, a membrane-electrode assembly for a fuel cell, and a fuel cell system including the same. More particularly, aspects of the present invention relate to a cathode catalyst having a high catalytic activity, and a membrane-electrode assembly and a fuel cell system including the same.
2. Description of the Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant and hydrogen, particularly hydrogen contained in a hydrocarbon-based material such as methanol, ethanol, or natural gas.
Such a fuel cell is a clean energy system that can replace energy systems that use fossil fuels. A typical fuel cell includes a stack composed of unit cells and produces various ranges of power output. Since a fuel cell has an energy density that is four to ten times higher than that of a small lithium battery, fuel cells have been promoted as small portable power sources.
Representative exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell that uses methanol as a fuel.
The polymer electrolyte fuel cell has advantages of high energy density and high power, but also has problems such as the need to handle hydrogen gas carefully and the requirement for accessory facilities such as a fuel reforming processor for reforming methane or methanol, natural gas, and the like in order to produce the hydrogen that is used as the fuel gas.
On the other hand, a direct oxidation fuel cell has a lower energy density than a polymer electrolyte fuel cell has, but has the advantages of easy handling, a low operation temperature, and no requirement of additional fuel reforming processors.
In the above-mentioned fuel cell systems, a stack that generates electricity typically includes several to scores of unit cells stacked adjacent to one another, and each unit cell is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly is composed of an anode (also referred to as a “fuel electrode” or an “oxidation electrode”) and a cathode (also referred to as an “air electrode” or a “reduction electrode”) that are separated by a polymer electrolyte membrane.
In a typical fuel cell system, fuel is supplied to an anode and is adsorbed onto catalysts of the anode, where the fuel is oxidized to produce protons and electrons. The electrons are transferred to a cathode via an external circuit, thereby producing usable electricity, and the protons are transferred into the cathode through the polymer electrolyte membrane. In addition, an oxidant is supplied to the cathode. The oxidant, protons, and electrons react in the presence of catalysts of the cathode to produce water.